The invention relates to a method and device for determining the change in thickness and/or orientation within a strip of optically active material by measuring the intensity of light shining through the strip of material.
In the production of films from thermoplastic materials, polymer granulate is formed into a molten film by means of an extruder and a slit die. This film solidifies on contact with chill rolls to form a precursor film, which is then stretched in steps or simultaneously by means of stretching machines. During the stretching process, the molecule chains in the film material are oriented, which is essentially caused by the stretching process. The stretching process is carried out initially in or transverse to the machine direction (longitudinal or transverse stretching, respectively) and subsequently in the direction that the film was not initially stretched (transverse or longitudinal stretching, respectively). The transverse stretching is generally carried out in a frame. After completion of the stretching, the film strip is subjected to temperature treatment, by means of which the aligned molecule chains in the film are thermally fixed; the shape of the film material is thus maintained. A partially crystalline film, preferably biaxially oriented, is obtained through the fixing.
The orientation within the film can be described by the index ellipsoid (indicatrix), which is defined by the refractive indices n.sub.1, n.sub.2 and n.sub.3. Different refractive indices in three axial directions perpendicular to one another result in a correspondence to the orientation of the molecule chains in the film. The refractive index which is parallel to the machine direction is designated n.sub.1, the index at 90.degree. with respect to the machine direction and in the film plane is designated n.sub.2, and the index in the direction perpendicular to the film plane is designated n.sub.3. If a biaxially oriented film is introduced into a polarization arrangement, the polychromic or white light of a light source, after passing through the polarization arrangement, gives a characteristic interference color and intensity corresponding to the thickness and orientation of the molecule chains of the film web.
From German Patent No. 2,338,305, a method is known for the determination of the linear birefringence of an optically active material, in which the material is transilluminated with linearly polarized light and the light emerging in a polarization plane perpendicular to the polarization plane of the incident light is detected, at least one wavelength being determined at which the detected light is extinguished. The measuring device used for this comprises a light source whose beam transilluminates a polarizer in which the required linearly polarized wave is formed. This wave then passes through the film to be measured and reaches an analyzer. From there, the wave passes into a detector system. The detector system may be designed as a prism or grating, or as an optical multichannel analyzer having a large number of detectors. The light source emits monochromatic or white light.
From German Offenlegungsschrift No. 3,106,818, a method is known for the continuous determination of multiaxial orientation conditions of stretched films or sheets via their main birefringence values, in which three laser beams, generated by three lasers or by splitting one laser beam into three sub-beams, are used. One beam passes vertically through the film and the other two pass through it inclined at such an angle that the planes of inclination are perpendicular to the film plane and contain the two main orientation directions. The phase differences of the laser beam intensities are measured continuously after the beams pass through the film using a quarter-wave plate and a rotating analyzer. The three main birefringence values of the film are determined continually from the three phase differences, taking into account the two angles of inclination of the inclined laser beams and the film thickness measured in another fashion. The measuring device comprises three lasers whose beams are aligned parallel to each other by a suitable lens or mirror arrangement after passing through the film. The light beam of the first laser is incident perpendicularly on the film plane, while the light beams of the two other lasers are incident on the film at an angle .PHI. with respect to the film normals. The light beam of the first laser proceeds in a plane which contains the film normals and the main stretching direction of the film, while the laser beam of the other laser is in a plane which is determined by the film normals and the transverse stretching direction of the film. The light beams, initially linearly polarized, proceeding from the lasers are elliptically polarized by the optical anisotropy of the stretched film. A quarter-wave plate below the film converts the elliptical polarization of the three laser beams into linear polarization. A rotating polarizing filter below the quarter-wave plate extinguishes the beams if their polarization directions are perpendicular to the polarization direction of the polarizing filter. The intensities of the laser beams are converted by light-sensitive detectors into periodical electrical signals which are phase-displaced to one another. This phase displacement can be determined using two phase meters, the third phase displacement making the other two add up to 0.degree.. The birefringence values can be determined from the measured phase displacements using a computer and used directly as measured quantities for the biaxial film orientation for control of the film-stretching equipment.
This known measuring device is instrumentally complicated due to the fact that it uses three lasers or an optical system to split a single laser beam into three sub-beams.
If the film has areas in which the orientation of the molecule chains in or transversely to the machine direction or the thickness are different compared to the surrounding areas, n.sub.1, n.sub.2, or the film thickness d changes, and another path difference .GAMMA.=(n.sub.1 -n.sub.2)d results in these areas. An alteration of the interference color and the intensity is connected with this.
Areas of lesser or greater orientation in the film can be produced as a result of localized thick or thin points in the precursor film. These areas are deformed to an essentially lesser or greater extent during the stretching process. If the thick or thin points in the precursor film have the shape of a streak several millimeters in width and up to several meters in length, the film resulting from the stretching process will also have streak areas of lesser or greater orientation with a width of several millimeters and a length of several meters. The mechanical properties (E module, .sigma..sub.5 -value) of the film correlate to the orientation. Streaks having lesser or greater orientation thus also exhibit lesser or greater mechanical properties. The appearance of such streaks thus leads to a reduction in quality of the film, which must be detected quickly.